This invention is in the field of drive and drive controllers for sucker-rod-pumped oil wells, particularly long-stroke pumping units.
Long-stroke pumping units are designed to lift great loads efficiently. Such units often employ belt-and-pulley systems, which include a flat belt that is coupled at one end to a counterweight assembly, that passes over a roller pulley near the top of the derrick, and that is coupled at the other end to the polished rod and rod string. A motor reciprocates the counterweight, and therefore the rod string, which drives the downhole pump. Examples of such pumping units are disclosed in my co-pending application Ser. No. 07/725,200, filed Jul. 3, 1991, and in U.S. Pat. No. 4,916,959, issued to Lively and assigned to a common assignee, both of which are incorporated herein by reference.
Mechanical, long-stroke pumping units often have a mechanical reversing mechanism that may suffer damage if the speed of the reversal at each end of the stroke is too rapid. That mechanical limitation constrains the operating speed of the pump to approximately 4 or 4.5 strokes per minute ("SPM"), which often limits the unit's ability to produce high volumes of fluid. If the pumping speed could be raised, greater production rates could be achieved.
It is therefore an object of the invention to increase pump production by varying pumping speed.
It is another object of the invention to avoid damage to the pumping equipment by controlling the speed of the pump motor depending on factors such as pump position, load, and type of fluid being pumped.
It is another object of the invention to increase pump speed in portions of the stroke that will permit safe increase in production.
It is another object of the invention to reduce pump speed in the portions of the stroke that are susceptible to equipment damage.
It is another object of the invention to improve production of viscous fluids by increasing pumping speed on the upstroke.
It is another object of the invention to provide a means for allowing the operator to control the pumping speed within differing parts of a stroke.
It is another object of the invention to provide a pump that works at a different speed on the upstroke than on the downstroke.
The above and other objects are achieved by an apparatus and method for controlling a pump lifting system. A lifting string is vertically reciprocated through a pumping stroke. A variable-speed drive is controlled through an electronic feedback loop from the shaft of the motor, back to a drive-control system. Position sensors detect the passage of some portion of the apparatus past a fixed position in the pumping stroke and send signals to the drive controller. The signals from the position sensors are used to vary the motor speed, thus controlling the pumping speed. The controller may operate to slow down at both ends of the long stroke, to have different speeds on the upstroke or downstroke, or to permit manual or automated variations in the pumping speed over time. By utilizing such a drive in a long-stroke pumping unit, the average pumping speed of the unit can be safely increased without equipment damage.
Other aspects of the invention will be appreciated by those skilled in the art after a reading of the detailed disclosure of the present invention below.